ML25087A090

Issuance of Amendment Nos. 176 and 81 Regarding the Revised Hydrologic Analysis

ML25087A090
Person / Time
Site:	Watts Bar
Issue date:	05/08/2025
From:	Kimberly Green Plant Licensing Branch II
To:	Erb D Tennessee Valley Authority
References
EPID L-2024-LLA-0006
Download: ML25087A090 (1)

Text

CEII - DO NOT RELEASE CEII - DO NOT RELEASE May 8, 2025 Mr. Delson C. Erb Vice President, OPS Support Tennessee Valley Authority 1101 Market Street, LP 4A-C Chattanooga, TN 37402-2801

SUBJECT:

WATTS BAR NUCLEAR PLANT, UNITS 1 AND 2 - ISSUANCE OF AMENDMENT NOS. 176 AND 81 REGARDING THE REVISED HYDROLOGIC ANALYSIS (EPID L-2024-LLA-0006)

Dear Mr. Erb:

The U.S. Nuclear Regulatory Commission (the Commission) has issued enclosed Amendment No. 176 to Facility Operating License No. NPF-90 and Amendment No. 81 to Facility Operating License No. NPF-96 for the Watts Bar Nuclear Plant (Watts Bar), Units 1 and 2, respectively.

These amendments are in response to your application dated December 26, 2023, as supplemented by letter dated November 20, 2024.

The amendments revise the Watts Bar Dual-Unit Updated Final Safety Analysis Report to reflect the results from a new hydrologic analysis for the Watts Bar site.

The staff has determined that Enclosure 3 contains potential Critical Electric Infrastructure Information (CEII) as well as proprietary information. Accordingly, the staff has prepared a redacted version, which is provided as Enclosure 4. CEII and proprietary text will be contained within double brackets (( this is CEII or proprietary text )).

to this letter contains potential CEII information and proprietary information. When separated from Enclosure 3, this document is decontrolled.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE A copy of our related safety evaluation is also enclosed. A notice of issuance will be included in the Commissions monthly Federal Register notice.

Sincerely,

/RA/

Kimberly J. Green, Senior Project Manager Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-390 and 50-391

Enclosures:

1. Amendment No. 176 to NPF-90

2. Amendment No. 81 to NPF-96

3. Safety Evaluation cc: Listserv

CEII - DO NOT RELEASE TENNESSEE VALLEY AUTHORITY DOCKET NO. 50-390 WATTS BAR NUCLEAR PLANT, UNIT 1 AMENDMENT TO FACILITY OPERATING LICENSE Amendment No. 176 License No. NPF-90

1.

The U.S. Nuclear Regulatory Commission (the Commission) has found that:

A.

The application for amendment by Tennessee Valley Authority (TVA, the licensee) dated December 26, 2023, as supplemented by letter dated November 20, 2024, complies with the standards and requirements of the Atomic Energy Act of 1954, as amended (the Act), and the Commissions rules and regulations set forth in 10 CFR Chapter I; B.

The facility will operate in conformity with the application, the provisions of the Act, and the rules and regulations of the Commission; C.

There is reasonable assurance (i) that the activities authorized by this amendment can be conducted without endangering the health and safety of the public, and (ii) that such activities will be conducted in compliance with the Commissions regulations; D.

The issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public; and E.

The issuance of this amendment is in accordance with 10 CFR Part 51 of the Commissions regulations and all applicable requirements have been satisfied.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE

2.

Accordingly, by Amendment No. 176, the license is amended to authorize revision to the Updated Final Safety Analysis Report (UFSAR), as set forth in the application dated December 26, 2023, as supplemented by letter dated November 20, 2024.

3.

This license amendment is effective as of the date of its issuance and shall be implemented within 60 days.

FOR THE NUCLEAR REGULATORY COMMISSION

/RA/

David Wrona, Chief Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Date of Issuance: May 8, 2025

CEII - DO NOT RELEASE CEII - DO NOT RELEASE TENNESSEE VALLEY AUTHORITY DOCKET NO. 50-391 WATTS BAR NUCLEAR PLANT, UNIT 2 AMENDMENT TO FACILITY OPERATING LICENSE Amendment No. 81 License No. NPF-96

1.

The U.S. Nuclear Regulatory Commission (the Commission) has found that:

A.

The application for amendment by Tennessee Valley Authority (TVA, the licensee) dated December 26, 2023, as supplemented by letter dated November 20, 2024, complies with the standards and requirements of the Atomic Energy Act of 1954, as amended (the Act), and the Commissions rules and regulations set forth in 10 CFR Chapter I; B.

The facility will operate in conformity with the application, the provisions of the Act, and the rules and regulations of the Commission; C.

There is reasonable assurance (i) that the activities authorized by this amendment can be conducted without endangering the health and safety of the public, and (ii) that such activities will be conducted in compliance with the Commissions regulations; D.

The issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public; and E.

The issuance of this amendment is in accordance with 10 CFR Part 51 of the Commissions regulations and all applicable requirements have been satisfied.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE

2.

Accordingly, by Amendment No. 81, the license is amended to authorize revision to the Updated Final Safety Analysis Report (UFSAR), as set forth in the application dated December 26, 2023, as supplemented by letter dated November 20, 2024.

3.

This license amendment is effective as of the date of its issuance and shall be implemented within 60 days.

FOR THE NUCLEAR REGULATORY COMMISSION

/RA/

David Wrona, Chief Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Date of Issuance: May 8, 2025

CEII - DO NOT RELEASE CEII - DO NOT RELEASE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NOS. 176 AND 81 TO FACILITY OPERATING LICENSE NOS. NPF-90 AND NPF-96 TENNESSEE VALLEY AUTHORITY WATTS BAR NUCLEAR PLANT, UNITS 1 AND 2 DOCKET NOS. 50-390 AND 50-391

1.0 INTRODUCTION

By application dated December 26, 2023 (Reference 1), as supplemented by letter dated November 20, 2024 (Reference 2), the Tennessee Valley Authority (TVA, the licensee),

submitted a license amendment request (LAR) for Watts Bar Nuclear Plant (Watts Bar or WBN),

Units 1 and 2. The requested changes would revise the Watts Bar Dual-Unit Updated Final Safety Analysis Report (UFSAR), section 2.4, Hydrologic Engineering, and related tables and figures to reflect the revised hydrologic analysis that utilizes the probable maximum precipitation (PMP) methodology provided in Topical Report TVA-NPG-AWA16-A, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041 (Reference 0), including changes in the PMP used in the local intense precipitation (LIP) and the rivers and streams flooding models; revision of the geometry and reservoir overbank storage in the corrected Hydrologic Engineering Center River Analysis System (HEC-RAS) model; updated wind speed used in the wind wave analysis; updated seismically induced dam failure flooding analysis to current U.S. Nuclear Regulatory Commission (NRC or the Commission) Japan Lessons-Learned Project Directorate (JLD) guidance, NRC JLD-ISG-2013-01, Guidance For Assessment of Flooding Hazards Due to Dam Failure (Reference 0); and revision of the warning time plan resulting from these changes. The requested changes would also delete UFSAR Appendix 2.4A, SOCH [Simulated Open Channel Hydraulics] Model, due to the replacement of the SOCH software with the HEC-RAS software.

The supplement dated November 20, 2024, provided additional information that clarified the application, did not expand the scope of the application as originally noticed, and did not change the staffs original proposed no significant hazards consideration determination as published in the Federal Register on April 2, 2024 (89 FR 22754).

CEII - DO NOT RELEASE CEII - DO NOT RELEASE

2.0 REGULATORY EVALUATION

2.1 Background

In November 2015 the licensee identified a potential error in the WBN hydrologic flooding simulation using the United States Army Corps of Engineers (USACE) HEC-RAS model (Reference 0). This potential error could result in an overestimation of flood storage capacity in reservoirs within the HEC-RAS model and an underestimation of flooding levels at critical dams and at the WBN site. Subsequent communication between the licensee and the USACE confirmed that the method used by the licensee in the calculation of overbank storage was in error.

As part of the corrective action plan for the HEC-RAS volume issue, the licensee submitted Topical Report (TR) TVA-NPG-AWA16, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation [LIP] Analysis, Calculation CDQ0000002016000041, to the NRC for review and approval (Reference 0). The licensee provided a revised TR TVA-NPG-AWA16 to the staff (Reference 7). The NRC formally approved the TR in a safety evaluation (SE) report on March 18, 2019 (Reference 8). On May 21, 2019 (Reference 9), the licensee provided the approved proprietary and non-proprietary versions of TR TVA-NPG-AWA16-A to the NRC. On July 3, 2019 (Reference 0), the NRC staff verified that the licensee met the requirements for publishing approved proprietary and non-proprietary versions of the TR and determined that the submitted -A versions are acceptable for referencing in licensing applications for nuclear power plants to the extent specified and under the limitations and conditions delineated in the accepted versions of the TR.

In September 2020 and later in May 2021, while updating the WBN HEC-RAS flooding simulation model to address the volume issue, the licensee identified additional issues with dam stability assumptions supporting the results and conclusions of the PMF analysis in the WBN UFSAR. These issues were documented in the TVA Corrective Action Program and involved (1) an assumption of dam stability at the Apalachia Dam for a headwater and tailwater elevation not supported by an analysis and (2) an assumption of dam stability of several embankment supported spillways at flood water flows in excess of the original design assumptions.

To resolve these HEC-RAS model issues, the licensee used the updated PMP provided in TR TVA-NPG-AWA16-A, the corrected HEC-RAS model geometry and overbank reservoir storage, and updated stability assumptions for Apalachia Dam and affected embankment supported spillways to revise the stream course hydrologic models used for predicting flooding impacts at the TVA nuclear plants. In addition, the licensee used the updated PMP to revise the LIP analysis. The licensee also updated the seismically induced dam failure flooding analysis to replace the TVA SOCH software with HEC-RAS software and applying the current NRC guidance provided in NRC JLD-ISG-2013-01.

2.2 Proposed Changes to UFSAR The licensee proposed changes to the WBN UFSAR, section 2.4, Hydrologic Engineering, related tables and figures, and Appendix 2.4A, SOCH Model, to reflect changes in the PMP used in the LIP and the rivers and streams flooding models, revision of the geometry and reservoir overbank storage in the HEC-RAS model, update of wind speed used in the wind wave analysis, update of seismically induced dam failure flooding analysis to current NRC

CEII - DO NOT RELEASE CEII - DO NOT RELEASE guidance, and revision of the warning time plan resulting from these changes. The proposed detailed changes are shown in the form of a mark-up to the UFSAR, which can be found in to the LAR.

2.3 Applicable Regulatory Requirements and Guidance Under Title 10 of the Code of Federal Regulations (10 CFR) 50.90, Application for amendment of license, construction permit, or early site permit, whenever a holder of a license wishes to amend the license, including TSs in the license, an application for amendment must be filed, fully describing the changes desired. Under 10 CFR 50.92(a), determinations on whether to grant an applied-for license amendment are to be guided by the considerations that govern the issuance of initial licenses to the extent applicable and appropriate. Both the common standards for licenses in 10 CFR 50.40(a), and those specifically for issuance of operating licenses in 10 CFR 50.57(a)(3), provide that there must be reasonable assurance that the activities at issue will not endanger the health and safety of the public, and that the applicant will comply with the Commissions regulations.

The regulation at 10 CFR 100.10, Factors to be considered when evaluating sites, requires, in part, that physical characteristics of the site, including hydrology, be considered as part of the acceptability of the site.

The regulation at 10 CFR 100.23(d) sets forth the design criteria for geological and seismic siting factors with respect to seismically induced floods and water waves at the site.

Appendix A, General Design Criteria [GDC] for Nuclear Power Plants, to 10 CFR Part 50, Domestic Licensing of Production and Utilization Facilities, establishes the minimum requirements for the principal design criteria for water-cooled nuclear power plants. The principal design criteria establish the necessary design, fabrication, construction, testing, and performance requirements for structures, systems, and components important to safety.

According to section 3.1.1 of the WBN UFSAR, the plant was designed to meet the intent of the Proposed General Design Criteria for Nuclear Power Plant Construction Permits, published in July 1967. The WBN construction permits were issued in January 1973. The WBN plant, in general, meets the intent of the NRC GDC published as Appendix A to 10 CFR Part 50 in July 1971, as discussed in UFSAR section 3.1.2 (Reference 0). The NRC staff considered the following GDC as part of its review:

GDC 2, Design bases for protection against natural phenomena, states, in part, that the design bases for structures, systems, and components (SSCs) shall reflect appropriate consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated.

GDC 44, Cooling water, states, in part, that an ultimate heat sink shall be provided for normal operating and accident conditions.

The regulation at 10 CFR 50.34(b)(6)(ii) requires information to be provided regarding the managerial and administrative controls to be used to assure safe operation, including a discussion of how applicable requirements within Appendix B, to 10 CFR Part 50 are satisfied.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE The regulation at 10 CFR Part 21, Reporting of Defects and Noncompliance, Section 21.3, Definitions, defines the term dedication, in part as, an acceptance process undertaken to provide reasonable assurance that a commercial grade item to be used as a basic component will perform its intended safety function and, in this respect, is deemed equivalent to an item designed and manufactured under a 10 CFR Part 50, appendix B quality assurance programIn all cases, the dedication process must be conducted in accordance with applicable provisions of 10 CFR Part 50, appendix B. The NRC staff considered the below applicable criteria in Appendix B to 10 CFR Part 50:

Criterion II, Quality Assurance Program, which requires, in part, that The applicant shall identify the structures, systems, and components to be covered by the quality assurance program....

Criterion III, Design Control, which requires, in part, that Measures shall also be established for the selection and review for suitability of application of materials, parts, equipment, and processes that are essential to the safety-related functions of

[SSCs]...The design control measures shall provide for verifying or checking the adequacy of design, such as by the performance of design reviews, by the use of alternate or simplified calculational methods, or by the performance of a suitable testing program.

Criterion VII, Control of Purchased Material, Equipment, and Services, which requires, in part, that measures be established to assure that purchased material, equipment, and services, whether purchased directly or through contractors and subcontractors, conform to the procurement documents...Documentary evidence that material and equipment conform to the procurement requirements shall be available at the nuclear powerplant or fuel reprocessing plant site prior to installation or use of such material and equipment.

The NRC staff considered the applicable regulatory guidance below which provide one acceptable means to meet the above stated regulations:

Regulatory Guide (RG) 1.27, Ultimate Heat Sink for Nuclear Power Plants, describes an acceptable basis that can be used to meet GDC 44 and 2 regarding the ultimate heat sink.

RG 1.28, Quality Assurance Program Criteria (Design and Construction), Revision 4, endorses, with clarifications and regulatory positions, the American Society of Mechanical Engineers (ASME) Nuclear Quality Assurance (NQA)-1-2008, Quality Assurance Requirements for Nuclear Facility Applications, standard and the NQA-1a-2009 Addenda1.

RG 1.29, Seismic Design Classification for Nuclear Power Plants, provides an acceptable method for use in identifying and classifying those features of light-water 1 Topical Report TVA-NPG-AWA16-A, Revision 1 referenced in NQA-1-208 as the quality assurance standard used.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE reactor (LWR) nuclear power plants that must be designed to withstand the effects of the safe-shutdown earthquake.

RG 1.59, Design Basis Floods for Nuclear Power Plants, discusses the design basis floods (DBFs) that nuclear power plants should be designed to withstand without loss of capability for cold shutdown and maintenance thereof.

RG 1.102, Flood Protection for Nuclear Power Plants, describes types of flood protection acceptable to the staff for the SSCs identified in RG 1.29. In addition, it describes acceptable methods of protecting nuclear power plants from the effects of PMP falling directly on the site.

RG 1.164, Dedication of Commercial-Grade Items for Use in Nuclear Power Plants, endorses, in part, the Electric Power Research Institute (EPRI) 3002002982, Revision 1 to EPRI NP-5652 and Technical Report (TR)-102260, Plant Engineering: Guideline for the Acceptance of Commercial-Grade Items in Nuclear Safety-Related Applications, with respect to acceptance of commercial-grade dedication of items and services to be used as basic components for nuclear power plants. EPRI NP-5652 and TR-102260, Revision 1, provides guidance that can be used by dedicating entities (licensees and nuclear suppliers) to dedicate commercial-grade items for use in safety-related applications.

RG 1.231, Acceptance of Commercial-Grade Design and Analysis Computer Programs Used in the Safety-Related Applications for Nuclear Power Plants, endorses Revision 1 of the EPRI TR-1025243, Plant Engineering: Guideline for the Acceptance of Commercial-Grade Design and Analysis Computer Programs Used in Nuclear Safety-Related Applications, with respect to acceptance of commercial-grade design and analysis computer programs associated with basic components for nuclear power plants. EPRI TR-1025243, Revision 1, provides a methodology for safety classification of non-process computer programs that are not resident or embedded plant SSCs. This TR provides guidance for using commercial-grade dedication methodology to accept commercially procured computer programs that perform a safety-related function, including examples of critical characteristics for software and methods for verifying these critical characteristics that are consistent with the methods described in EPRI 3002002982.

NRC JLD-ISG-2013-01, Guidance For Assessment of Flooding Hazards Due to Dam Failure, provides guidance acceptable to the staff for re-evaluating flooding hazards due to dam failure as described in NRCs March 12, 2012, request for information (Reference 0) regarding Recommendation 2.1 of the enclosure to SECY-11-0093, Recommendations for Enhancing Reactor Safety in the 21st Century, the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident (Reference 0).

NUREG/CR-7046, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America (Reference 0), discusses the sensitivities of the rainfall-to-runoff transformation for peaking and lagging the unit hydrographs.

NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition, establishes criteria that staff responsible for the review of

CEII - DO NOT RELEASE CEII - DO NOT RELEASE applications to construct and operate nuclear power plants intend to use in evaluating whether an applicant/licensee meets the NRCs regulations. The following sections are applicable to the NRC staffs review:

Section 2.4.1, Hydrologic Description Section 2.4.2, Floods Section 2.4.3, Probable Maximum Flood (PMF) on Streams and Rivers Section 2.4.4, Potential Dam Failures Section 2.4.10, Flooding Protection Requirements Section 2.4.14, Technical Specifications and Emergency Operation Requirements

3.0 TECHNICAL EVALUATION

The NRC staff evaluated the licensees application to determine whether the proposed changes to the WBN UFSAR are consistent with the regulations, guidance, and plant-specific design and licensing basis information discussed in section 2.3 of this SE.

The NRC staff reviewed the acceptability of the proposed changes to hydrologic analysis and the UFSAR by evaluating whether, among other things, the changes provide reasonable assurance of public health and safety.

Site Description The WBN site is situated along the west bank of Chickamauga Lake at Tennessee River Mile 528, approximately 1.9 miles downstream of Watts Bar Dam. The site covers approximately 1,770 acres and has a plant grade elevation of 728.0 feet (ft) mean sea level.

The Watts Bar Dam has a drainage area of 17,310 square miles, whereas the drainage area above Chickamauga Dam (57 miles downstream of WBN) is 20,790 square miles. Several major tributaries reside within the drainage area, which includes the French Broad, Holston, Little Tennessee, Clinch, and Hiwassee Rivers (Reference 1).

The climate of the WBN site watershed is humid and temperate with annual average rainfall of 50 inches observed upstream of the Watts Bar Dam. Most floods at the WBN site have occurred from winter-type storms during the major flooding seasons in the months of January through early April. There are 12 major dams upstream of the WBN site, 10 of which provide flood mitigation services without appreciable flood storage capacity. The Watts Bar Dam releases water from storage with an average daily discharge of 27,000 cubic feet per second (cfs).

During the winter, channel velocities at WBN average about 2.3 feet per second, whereas during the summer months the average velocity is lower at about 1.0 feet per second (Reference 1).

3.1 Evaluation of Probable Maximum Precipitation The licensee revised the PMP used for the simulation of LIP flooding and drainage analysis at the WBN site. The revised PMP was also used to simulate flooding in streams and rivers of the WBN watershed. The revised PMP is based on Topical Report TVA-NPG-AWA16-A. The updated PMP for the WBN rivers and streams rainfall flooding hydrologic analysis provides depth-area-duration characteristics for three storm types at each point of a gridded network over the Tennessee River drainage basin for three storm types: local, general, and tropical.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE The licensee revised its calculation for LIP at WBN (Reference 0). The NRC staff previously reviewed and approved the original LIP calculation, which set the value of LIP throughout the modeled basin, including at the location of TVAs three nuclear power plants (Reference 8). For WBN, the existing design basis local storm defines a cumulative 1-hour, 1 square mile PMP rainfall depth of 16.76 inches with early, middle, and late temporal distributions modeled. The updated PMP for the WBN site LIP drainage analysis for 1-hour, 1 square mile local intense PMP event is 13.81 inches. The licensee performed hydrologic modeling using early, middle, and late temporal LIP distributions. The results of the LIP flood model analysis show that maximum water surface elevations (WSEs) are not predicted to exceed the critical plant elevation of ((

)) at any modeled locations for the WBN site.

The NRC staff reviewed the licensees LIP flooding analysis including implementation of the LIP PMP depth used for the simulation model developed for basin area, the 1-hour, 1 square mile PMP rainfall distribution, temporal distribution of the PMP. The results of the review confirmed that the LIP induced flooding using the prescribed PMP input does not produce flood elevations that exceed ((

)) at modeled locations for the WBN site.

Because the licensee developed the updated LIP flooding analysis using the updated PMP from the NRC-approved topical report TVA-NPG-AWA16-A and the results do not produce a flood that exceeds the critical plant elevation, the NRC staff finds that the revised PMP analysis and the LIP analysis are acceptable.

3.2 Evaluation of HEC-RAS Model Geometry Changes The licensee updated the HEC-RAS model geometry for the reservoir above Douglas Dam on the French Broad and Nolichucky Rivers, for the reservoir and above Cherokee Dam on the Holston River and the downstream portion of the South Fork Holston River to more accurately characterize overbank storage through the additional cross-sections and storage areas, connected by lateral structures. Additionally, it proposed to adjust the overbank storage volumes in the other modeled reservoirs by using the average reach length in the overbank storage volume calculation.

Once the cross-sections and storage areas were developed, the licensee adjusted storage volumes iteratively beginning at the lowest elevation and working toward the highest elevation to retain the integrity of the overall volume of the system. The licensee computed volumes between each cross-section and summed with storage area volumes to get a cumulative volume at various water surface elevations and then compared the cumulative volumes to published reservoir volumes. For each volume (reservoir), the comparison showed that the calculated storage volumes matched the published reservoir volumes. For the ones that differed, the percentage difference was no more than -0.3 percent.

The updated hydrologic analysis performed by the licensee shows that the maximum WSE corresponding to PMF reduced from ((

)) to ((

)). The corresponding PMP-PMF DBF elevation for plant protection is estimated to be ((

)). Therefore, the licensee determined that the current flood protection measures do not need to be changed.

The NRC staff reviewed the licensees updated HEC-RAS model configuration including the updated model geometry to address errors related to overbank storage volume in the HEC-RAS

CEII - DO NOT RELEASE CEII - DO NOT RELEASE model. The NRC staff confirmed that the model results for the PMF-induced flooding produced water surface elevation that the staff finds reasonable and provides adequate freeboard (the difference between the top of the dam and the maximum WSE) for the protection of the dams.

Because the licensee developed the updated PMF flooding analysis using the updated PMP from the NRC-approved topical report TVA-NPG-AWA16-A and the updated model geometry, the NRC staff finds that the revised PMP analysis and the PMF analysis are acceptable.

3.3 Evaluation of Wind Speed Data Update The licensee updated meteorological wind speed data to include more current wind data and data sources from airports in Chattanooga, Knoxville, and Tri-Cities, Tennessee, in Asheville, North Carolina, and in Huntsville, Alabama. As a result of this change, the licensee updated critical fetch and wind wave effects for WBN critical structures.

In the updated analysis, the licensee used 2-year wind speed which was the Automated Surface Observing System (ASOS) surface 1-minute data from the National Oceanic and Atmospheric Administration National Climatic Data Center for five airport data stations surrounding the WBN site (Reference 0). The licensee noted that a 20-minute sustained wind is sufficient to cause wind wave activity, therefore, the 2-minute average wind speed data were analyzed to find the peak 20-minute average wind speed for each year at the WBN site.

The licensee determined the 2-year overland wind speed by statistically analyzing the calculated peak 20-minute resultant velocities for each year for each airport reporting site and determining an effective weight for the 2-year wind speed of each reporting station relative to the WBN site. The sum of the weighted wind speeds from each of the five airport reporting stations was taken as the 2-year overland wind speed for the WBN site. For a bounding WBN site flood elevation of ((

)), the overland 2-year wind speed for general structures at the WBN site, including the Auxiliary, Control, and Shield buildings, was 26.4 miles per hour (mph).

The result of the 2-year overland wind speed with respect to the topography limited critical fetch at the Diesel Generator Building was 17.0 mph. The Intake Pumping Station, analyzed with respect to the critical fetches from both the northeast and southwest directions, has a 2-year overland wind speed of 18.1 mph for the northeast direction and 23.8 mph for the southwest direction.

To account for the increase in wind speed over water, the licensee converted 2-year overland wind speeds to over water wind speeds to determine the effective fetch length based on guidance provided in the USACE Engineering Technical Letter (ETL) 1110-2-588 (Reference 0).

For a bounding WBN site flood elevation of ((

)), the resulting over water wind speed at the Diesel Generator Building was 19.3 mph, 21.1 mph for the northeast fetch of the Intake Pumping Station, 27.4 mph for the southwest fetch of the Intake Pumping Station, and 28.5 mph for the west face and 29.8 mph for the east face of the Auxiliary, Control, and Shield buildings.

The licensee also reviewed wind wave effects to evaluate whether the dam failures would result from overtopping. The wind wave effect presented by the licensee (Reference 0) was not considered for the failure timing, but rather for analyzing the postulated failures and routing of the compound flood waves. The licensee used this review for the consideration of overtopping failures but not for the timing of the failures across PMF simulations conducted.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE The NRC staff reviewed the licensees updated meteorological wind speed data, and the wind wave effects estimated using the updated wind speed data. The NRC staff also reviewed the licensees analysis of postulated dam failure as well as routing of compound flood waves which were used to postulate overtopping dam failure based on PMF simulations. The NRC staff determined that the methodology adopted and process followed by the licensee to estimate wind speed and incorporate the effects of wind wave into the analysis of overtopping dam failure are acceptable because they meet the accepted standards described in the guidance documents referenced in section 2.3 above, as well as the modeling and simulation recommended practices in the documentation of the modeling software used as described in section 2.1 above. Therefore, the NRC staff finds the combined effects consideration of PMF flooding with wind wave effects resulting in postulated overtopping dam failure to be reasonable.

The NRC staff evaluated the postulated dam failures set at peak WSEs across dams upstream of the Watts Bar Dam. The NRC staff performed a confirmatory analysis and confirmed the control simulation resulted in the postulated dam failures. Therefore, the staff finds the overtopping failure mode defined with the linear progression of dam breach and the control simulation results in the postulated upstream dam failures to be reasonable.

3.4 Evaluation of Flood Induced by PMP Combined with Hydrological Failures of Dams The licensee used the gridded precipitation (Reference 0) defined in Topical Report TVA-NPG-AWA16-A developed for the TVA basins to compute the overall basin PMP (Reference 0) and to perform hydrologic and hydraulic simulation modeling using the Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) and HEC-RAS, respectively. The model results provided simulated maximum flooding scenarios. The licensee used multiple PMP events over the watersheds considered and PMP events upstream of Fort Loudoun Dam (FLH) - Tellico Dam (TEC) to develop PMF simulations that lead to the candidate PMF water surface elevations for WBN and SQN, as well as Watts Bar and Chickamauga Dams. The licensee identified three PMP storm types: General, Local, and Tropical, with various corresponding temporal rainfall distributions that enable maximization of the consequent flood estimates at FLH - TEC (Reference 0). The licensee used a combination of 48-and 72-hour design storm distributions with three temporal distributions: front-loaded, median-loaded, and back-loaded, for both the main and antecedent storms to simulate PMFs (Reference 0). Based on the results of the simulation, the licensee concluded that general storm type PMP is the controlling storm type for FLH - TEC.

The licensee performed PMF simulations using multiple PMP nesting sequences and storm weeks (weeks 15, 18, and 19). The PMP results were used to perform hydraulic simulations using the validated HEC-RAS model with inputs from spatially varied storm events with respective temporal distributions between weeks 11 and 23. The procedure entails the entire modeling chain of PMF simulations executed through the PMP/PMF Calculation Automation Tool (PCAT) (Reference 0). Results of the control PMF simulation outputs for FLH - TEC and WBN are presented in table 1.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE Table 1. Control PMF simulation outputs for FLH - TEC and WBN

((

))

CEII - DO NOT RELEASE CEII - DO NOT RELEASE The licensee postulated overtopping failures of all earthen dams upstream of WBN for determining PMF water surface elevations and no failures for dams downstream of Watts Bar Dam and WBN. This postulated overtopping failure of all embankment dams is consistent with the guidance and the recommended practices for the analyses of dam failure induced flooding referenced in section 2.3 above, by considering a conservative flooding scenario at the WBN site. Therefore, the NRC staff considers this assumption to be reasonable for PMF evaluation at Watts Bar Dam and WBN. The NRC staff performed confirmatory simulations using 48-and 72-hour design storm distributions. The NRC staff reviewed the unsteady flow model configuration including the flow augmentation set to prevent model instabilities.

The NRC staff reviewed the licensees flood simulation and estimated water surface elevations by performing confirmatory analysis and finds the licensees analysis to be reasonable. The staff finds that the water volume check performed by the licensee to evaluate the mass conservation in the modeled river system to be reasonable based on the accepted standards of practice of simulation for flooding potential analyses, per the staffs engineering judgment. The staff also finds the number of simulations to be reasonable in reflecting the spatial and temporal variability of PMP storm events driving the HEC-RAS model for FLH - TEC (Reference 0). The analysis confirmed the validity of the applicants postulated scenarios and provides a reasonably conservative estimate of flood hazard against which the current protection measures are expected to work.

As stated in the previous paragraphs, the NRC staff has considered several factors to determine the acceptability of various components of the analyses for overtopping failure of dams.

Therefore, the NRC staff finds that the postulated failure of upstream dams due to overtopping with linear progression of dam breach, as confirmed by the control simulation, represents a reasonable flooding scenario against which the WBN site is protected; therefore, the NRC staff finds this analysis and the results to be acceptable.

3.5 Evaluation of Seismically Induced Dam Failure Flooding Analysis Changes The licensee formerly used procedures described in Appendix A of RG 1.59 to evaluate potential flood levels from seismically induced dam failures. The licensee evaluated potential seismic dam failure for two scenarios:

1. Safe shutdown earthquake (SSE) coincident with the peak of the 25-Year flood and a two-year wind speed applied in the critical direction.

2. Operating basis earthquake (OBE) coincident with the peak of the one-half PMF and a two-year wind speed applied in the critical direction.

Also, the epicenter of the OBE/SSE was moved to multiple locations above WBN to examine seismic attenuation and determine multi-dam impact (base accelerations) due to single seismic events. Dams that were identified to have been impacted in the attenuation study were further assessed to determine their seismic stability. The extent of flooding from seismic failure of dams at the WBN site was estimated by incorporating the results of the seismic stability analyses in the flooding simulations.

The licensee had previously performed unsteady flow simulations using the results of the dam seismic stability analyses. The unsteady hydraulic simulation was performed using TVAs

CEII - DO NOT RELEASE CEII - DO NOT RELEASE unsteady flow modeling software SOCH. The postulated dam failure-induced flood outflow hydrographs were obtained from simulations using the USACEs HEC-HMS, the SOCH, and HEC-RAS.

As described in the current UFSAR, the licensee analyzed a total of five postulated combinations of seismic dam failures combined with floods to estimate the maximum WSE resulting from the postulated SSE failure of ((

)) dams coincident with the 25-Year postulated flood occurring in the month of June. The estimated maximum WSE, excluding wind wave effects, from the postulated dam failure analysis at the WBN site is ((

)).

The licensee revised the seismic hazard and basis for analyzing seismic dam failures in combination with floods to be consistent with the guidance provided in NRC JLD-ISG-2013-01.

The licensee updated the seismically induced dam failure flooding analysis to consider the more severe of the following combinations as stated in the guidance:

1. Seismic hazard with an annual exceedance probability of 10-4 combined with a 25-Year flood.

2. Seismic hazard defined by one-half of the 10-4 annual exceedance probability ground motion combined with a 500-Year flood.

The licensee also updated the seismic hazard attenuation methods and multiple dam failure evaluation techniques to be consistent with the guidance provided in JLD-ISG-2013-01. The licensee stated that the proposed changes demonstrate adequate margin to the design analysis flood (DAF) elevations considered in the flooding protection of safety-related SSCs during external flooding events, and these changes verify the adequacy of the warning time for WBN for rainfall floods.

In the updated analysis, the licensee analyzed four postulated combinations of seismic dam failures to determine the maximum WSE at the WBN site. The estimated maximum WSE at the WBN site resulting from multiple seismic dam failures due to ground motion of one half of 10-4 annual frequency of exceedance seismic event coincident with a 500-year flood is ((

)).

The NRC staff reviewed the update seismic attenuation method used in the analyses of multiple dam failures and confirmed the DAF elevations are reasonable. The staff determined that flood level estimates from the dam failure analysis provide reasonable assurance that adequate safety margins are available, such that the DAF is not challenged. The NRC staff finds the postulated dam failure analyses for the postulated scenarios to be consistent with the guidance documents described in section 2.3 above, and in particular, JLD-ISG-2013-01. Therefore, the NRC staff finds the analysis and scenarios to be reasonable.

The NRC staff determined that the seismic failure mode and the seismic hazard attenuation methods and multiple dam failure evaluation techniques are consistent with the guidance provided in section 2.3 above, and the simulation results in the postulated dam failures are reasonable. Therefore, the staff finds the analysis performed by the licensee to assess seismically induced dam failure is acceptable.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE 3.6 Water Level at the Plant Site The NRC staff performed a confirmatory analysis to evaluate the flood water level at the site.

During the confirmatory analysis the staff noted that the exact river mile corresponding to WBN was not defined in the HEC-RAS model setup (Reference 0). Therefore, the staff incorporated an interpolated channel geometry the WBN location for the confirmatory analysis and performed the control simulation. The results show that corresponding PMF simulation outputs approximately reproduce the reported WSE of ((

)) which is comparable to the sensitivity analysis results presented in table 2.

Table 2. Results for Sensitivity of PMF Simulation Outputs to Time Step Used in the Unsteady Flow Analysis in HEC-RAS for FLH - TEC and WBN

((

CEII - DO NOT RELEASE CEII - DO NOT RELEASE

))

3.7 Evaluation of Warning Time Analysis Changes The licensee updated the PMP used in the warning time analysis based on Topical Report TVA-NPG-AWA16-A. The licensee also updated the target WBN site elevation used in the development of flood warning notifications to plant grade or 728.0 ft. Additionally, the licensee updated the warning scheme for the WBN site by utilizing average rainfall on the basins above the Chickamauga Dam to act as the trigger for the TVA River Management flood simulation models in the assessment of the potential impacts of flooding at the WBN site.

The licensee stated that the WBN warning plan for rivers and streams rainfall flooding consists of two stages. Stage I requires a minimum of 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> whereas Stage II requires a minimum of 17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br />. The licensee stated that this approach allows sufficient preparation time for operation in flooding mode. The licensee also stated that the warning plan provides four additional hours prior to a Stage I warning, thus allowing the TVA River Management Forecasting Center to provide initial warning by analyzing available rainfall data. The licensee used average rainfall on the ground above Chickamauga, along with weather forecasts and flood stream course modeling, to provide Stage I and Stage II protective warnings to WBN. The licensee removed the warning time analysis and response plan for seismically induced dam failure flooding because the resultant flooding from these events does not reach plant grade.

The licensees updated WBN hydrologic analysis shows that the maximum WSE corresponding to the PMF reduced from ((

)) to ((

)). The corresponding DBF elevation, which is the result of PMF and is the design basis for plant protection, is ((

)), suggesting that the prevailing flood protection measures are not required to be changed. Therefore, the NRC staff finds the result and subsequent conclusion regarding protection measures to be reasonable.

The licensee conducted preliminary analyses of flood warning times based on the candidate PMF simulation results for all dams, including those with combined seismic hazards and extreme flood events. The licensees analysis indicated that the results are sensitive to the spatiotemporal distribution of PMPs and the week of occurrence. Additional spatial nesting sequences for PMPs were considered by the licensee as part of the warning time calculations.

The PMP nesting combinations used to produce the PMF simulations resulting in the maximum WSE at WBN do not necessarily produce the shortest or controlling warning times as the resulting stage and flow hydrographs may not be the fastest rising hydrographs to reach the plant grade in the least amount of time. The licensee stated that the fastest rising flood occurs during a PMP when the 6-hour increments increase throughout the storm with the maximum 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> occurring in the last period. Furthermore, the licensee used the plant grade at WBN and

CEII - DO NOT RELEASE CEII - DO NOT RELEASE SQN as the target for warning time calculations for which wind-induced wave heights have a negligible effect. The NRC staff agrees with the process followed by the licensee to determine PMP flooding because it is consistent with the procedures described in the guidance documents listed in section 2.3 above. Therefore, the NRC staff finds the licensees determination acceptable.

The licensee computed warning times associated with multiple simulations and evaluated them against the required shortest warning times relevant for WBN. The warning plan includes modeling and communication (4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> minimum), Stages I (10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> minimum) and II (17 hours1.967593e-4 days <br />0.00472 hours <br />2.810847e-5 weeks <br />6.4685e-6 months <br /> minimum) with the total warning time of at least 31 hours3.587963e-4 days <br />0.00861 hours <br />5.125661e-5 weeks <br />1.17955e-5 months <br />. Stage I determines whether the rainfall and streamflow observed during this period require more critical steps to be taken during Stage II. Three PMF events produced the rainfall lower boundary decision limits for modeling and communication (4.22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br />), Stage I (27.29 hours3.356481e-4 days <br />0.00806 hours <br />4.794974e-5 weeks <br />1.10345e-5 months <br />), and Stage II (17.05 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />) which is comparable to the shortest warning times for WBN.

The NRC staff performed a sensitivity analysis on the effect of the time step as it is a key factor leading to the model instability for unsteady flow simulation and affects the consequent WSEs.

Per the HEC-RAS model and accepted hydraulic simulation practice, satisfying the Courant condition is critical, especially when simulating multiple dam failure scenarios for which a time step as low as 5 seconds might be required. The sensitivity analysis used time steps ranging from 1 minute (used in Reference 0) to 5 seconds for the control simulation at FLH - TEC and WBN. The analysis showed that the model is stable for all time steps below 1 minute. The consequent WSEs exhibited fluctuation of ~0.2 ft from 1 minute to 15 seconds; however, they showed a major drop in WSE below the 10 seconds time step. Based on the confirmatory and sensitivity analyses, the NRC staff finds that the reported WSE at WBN i.e., ((

)) is reasonable.

The documented control maximum WSE at WBN is ((

)) (Reference 0). The LAR documents an updated still PMF WSE of ((

)), an increase of 0.5 ft from the ((

))

reported in Reference 0. The PMF results summary documented in table 8.2 of Reference 0 attributes the 0.5 ft increase as accounting for the potential uncertainties associated with the inputs and models. While there are no specific criteria to evaluate it, based on engineering judgment and the complexities of modeling and uncertainties, the NRC staff finds that it is a fair assumption. Therefore, the NRC staff finds that the updated PMF WSE of ((

)) is reasonable because it accounts for uncertainties in the inputs and modeling and is bounded by the existing DBF elevation of ((

)) reported in current WBN UFSAR.

The NRC staff reviewed the licensees technical evaluation and support analyses and determined that the licensees conclusion that sufficient warning time would be available to initiate WBNs flood mode operation before consequent flooding exceeds the plant grade is reasonable. Therefore, the NRC staff finds the licensees evaluation of warning time to be acceptable.

3.8 Evaluation of Low Water Considerations The licensee stated that maintaining minimum water levels at the WBN plant is not a problem because of its location on the Chickamauga Reservoir. The licensee also stated that the high rainfall and runoff of the watershed and the regulation afforded by upstream dams assure minimum flows for plant cooling.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE The current WBN UFSAR states that the probable targeted minimum water level at the WBN plant is elevation ((

)) and this would occur in the winter flood season resulting from the operation of the Chickamauga Reservoir. Historical records indicate that the most severe drought in the history of the Tennessee Valley region occurred in 1925. Low flow frequency studies for the period 1874 - 1935, which is prior to regulation, indicate that there is a less than one percent chance that the 1925 observed minimum one-day flow of 3,300 cfs downstream at Chattanooga might occur in a given year. At the plant site the corresponding minimum one-day flow is estimated to be 2,700 cfs.

The licensee replaced the TVA SOCH model with an analysis using the USACE HEC-RAS software model of the Tennessee River System to determine the minimum water level and the minimum flow from Watts Bar Dam required to maintain a minimum elevation at the WBN site.

The results showed the Chickamauga pool elevation dropping to ((

)) in 28 hours3.240741e-4 days <br />0.00778 hours <br />4.62963e-5 weeks <br />1.0654e-5 months <br />. A steady release of at least 3,200 cfs at Watts Bar Dam is provided within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> of Chickamauga Dam failure to ensure that the water level recession at the WBN site does not drop below elevation ((

)) at the intake channel.

The NRC staff reviewed the licensees calculation in Enclosures 1 - 4 to the LAR to determine if the approach and inputs are reasonable. The staff confirmed that the previous model was based on the TVA software SOCH whereas the updated design basis model is based on the Tennessee River geometry of the updated PMF model. This model uses the river hydraulic simulation software HEC-RAS and simulates the potential loss of the downstream dam.

Based on the review noted above, the NRC staff determined that the licensees approach to evaluate low water considerations is appropriate. Therefore, the staff finds the conclusions related to the low water considerations are acceptable.

3.9 Technical Conclusion for Revised Hydrologic Analysis The NRC staff finds that the licensees hydrologic analyses for LIP, PMF, and dam failure-induced flooding are reasonable and that the results documented in the LAR provide reasonable assurance that adequate safety margins are available.

As documented in the NRC staffs evaluation presented in sections 3.1 through 3.8, the staff concludes that the licensees revised flood hazard analyses, which include changing methodologies, consider dam failures, determine the controlling storm event, revise the target elevation of flood warning, and update wind-induced wave heights, are technically acceptable to support the UFSAR revisions.

As documented in the NRC staffs evaluation presented in sections 3.1 through 3.8, the staff concludes that the licensees updated PMP flood event is a controlling flood hazard at the WBN site and is consistent with NUREG/CR-7046, RG 1.59, JLD-ISG-2013-01, and NUREG-0800 sections 2.4.1 through 2.4.4.

The NRC staff confirmed that the change in the design basis flood PMF elevation is within the margin of error for estimating water surface elevations from floods of this magnitude and that there is adequate margin to the current design basis flood value. Therefore, the NRC staff

CEII - DO NOT RELEASE CEII - DO NOT RELEASE concludes that the updated design basis is acceptable and the licensees flood protection measures continue to meet the acceptance criteria of NUREG-0800 sections 2.4.10 and 2.4.14.

In sum, the NRC staff concludes that the licensees flood hazard analysis presented in the LAR meets NRC requirements consistent with the guidance and other documents listed in section 2.3 of this SE.

3.10 Software Dedication 3.10.1 Background As described in Attachment A, Topical Report Probable Maximum Precipitation (PMP)

Evaluation Tool and Application Using Geographic Information System (GIS), of Enclosure 5 to the LAR, two software tools were used to develop the watershed average rainfall over the TVA sub-basins. These tools include the PMP Evaluation Tool and an automation tool that runs the ArcGIS and Quantum GIS (QGIS) software to create PMP depth estimates across various storm durations and areas at each TVA sub-basin.

As stated in the LAR, the licensee provided a similar hydrologic analysis LAR for Sequoyah Nuclear Plant (Sequoyah or SQN), Units 1 and 2 (Reference 0). The NRC staff approved the hydrologic analysis LAR for SQN, Units 1 and 2, as documented in the staffs SE (Reference 0).

In section 2.0 of the SE, the staff reviewed the PMP Evaluation Tool, ArcGIS and QGIS software, and the automated scripts used to run these tools and concluded that:

The commercial-grade dedication activities of the PMP Evaluation Tool meet the requirements of Criterion II, III, and VII of Appendix B Quality Assurance Program Criteria for Nuclear Power Plants and Reprocessing Plants, to 10 CFR Part 50; and The design review activities performed for the ArcGIS and the Python scripts and Excel templates created to automate the PMP computation process meet the requirements of Criterion III of Appendix B to 10 CFR Part 50.

Attachment C, Applicability of TVA Submittals Responding to NRC Questions Relative to the Sequoyah LAR TS-19-02, of Enclosure 5, Evaluation of Proposed Changes (Public), of the WBN, Units 1 and 2, hydrologic analysis LAR, identified the responses to requests for additional information (RAIs) from the SQN, Units 1 and 2, hydrologic analysis LAR that are applicable to the WBN, Units 1 and 2, LAR. Relevant to the PMP Evaluation Tool, ArcGIS and QGIS, and automated scripts to run these tools, the following RAI 2 response and submitted document were identified as fully applicable to the WBN, Units 1 and 2, hydrologic analysis LAR:

TVA Letter to the NRC, CNL-20-032, Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis - Response to Request for Additional Information (TS-19-02) (Reference 0),

hereafter referred to as the RAI response; and TVA Letter to the NRC, CNL-21-095, Application to Revise Sequoyah Nuclear Plant, Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis - Software Dedication Report [(SDR)] 16 Update to Revision 4 (TS-19-02) (Reference 0) (PMP Evaluation Tool SDR).

CEII - DO NOT RELEASE CEII - DO NOT RELEASE The NRC staffs evaluation of the WBN, Units 1 and 2, hydrologic analysis LAR with respect to the PMP Evaluation Tool, ArcGIS and QGIS software, and the automated scripts used to run these tools is described in the sections below.

3.10.2 Technical Evaluation of Software Dedication Different methods are used to validate the acceptability of software tools that are used to perform analyses to establish design requirements of safety-related SSCs. For this LAR, the hydrologic analyses results derived from the output of the software tools, including the PMP Evaluation Tool, ArcGIS and QGIS, and the scripts used to run these tools, will be used to support establishing design requirements for flood protection of safety-related SSCs at WBN, Units 1 and 2. The summary of the methods used by the licensee to validate the acceptability of each of these software tools and the NRC staffs SE for each software tool is described below.

3.10.2.1 PMP Evaluation Tool Attachment A of Enclosure 5 to the WBN LAR states, in part, that the PMP Evaluation Tool computes the depth-area-duration (DAD) values for a defined watershed at grid points at set intervals of area and duration across the Tennessee Valley watershed above Kentucky Dam and the Great Falls project basin. The tool performs this computation using inputs of the storm type, storm duration, and selected areas from a user and DAD table lookups. The DAD table values used were approved storm data values for the TVA sub-basins which were evaluated in the SE for Topical Report TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis Calculation CDQ0000002016000041 (Reference 8).

Attachment A of Enclosure 5 of the LAR states, in part, that the PMP Evaluation Tool was used, as provided, as part of commercial-grade dedication, with some minor modifications. These modifications include removal of the capability to create raster files, removal of the script used to create TVA storm data, removal of the capability to extrapolate beyond the storm area range in each Storm Precipitation Analysis System (SPAS) event file, and password protecting the toolbox to prevent the user from modifying the tool.

Section 3 of the PMP Evaluation Tool SDR (Reference 0) states, in part, that the PMP Evaluation Tool package has been classified as safety-related since it ((

)) Section 4 of the PMP Evaluation Tool SDR states, in part, that ((

)) Table 1 in Attachment 5 of the SDR contains ((

))

Section 5 of the PMP Evaluation Tool SDR describes the failure modes and effects analysis approach and results and states, in part:

CEII - DO NOT RELEASE ((

Sections 6 and 7 of the PMP Evaluation Tool SDR describe the series of test case scenarios to verify and validate the functions of the PMP Evaluation Tool. Section 7 states, in part:

((

))

The NRC staff evaluated the information in the WBN, Units 1 and 2, hydrologic analysis LAR and the identified applicable information (i.e., RAI response and PMP Evaluation Tool SDR) from the SQN, Units 1 and 2, hydrologic analysis LAR. The staff determined that the technical basis for the staffs conclusion in section 2.3.1 of the SE for the SQN, Units 1 and 2 (Reference 0), hydrologic analysis LAR, with respect to the PMP Evaluation Tool, is the same as the information presented in the WBN, Units 1 and 2, hydrologic analysis LAR. As such, the NRC staff finds that dedication activities and results of the PMP Evaluation Tool are consistent with the definition of dedication in 10 CFR Part 21 and meet Criterion II, Criterion III, and Criterion VII of Appendix B to 10 CFR Part 50.

3.10.2.2 ArcGIS and QGIS The ArcGIS and QGIS software libraries are used to create PMP depth estimates across various storm durations and areas at each TVA sub-basin. The licensee stated in the RAI response that calculations made within ArcGIS outside of the PMP Evaluation Tool were checked via alternate methodologies within QGIS by comparison of the average basin rainfall depth determined by the two different GIS methodologies (software libraries) for specific grid locations within the sub-basin area. In the RAI response, the licensee also identified differences between ArcGIS and QGIS to demonstrate that two software tools are sufficiently diverse such that a common error in the software is unlikely and, therefore, allows the output of one tool to validate the output of the other tool.

The NRC staff evaluated the licensees method for validating the ArcGIS software library for compliance with Criterion III and Criterion VII to Appendix B of 10 CFR Part 50. The staff determined that the technical basis for the staffs conclusion in section 2.3.2 of the SE for the SQN, Units 1 and 2, hydrologic analysis LAR (Reference 0), with respect to the use of ArcGIS and QGIS, is the same as the information presented in the WBN, Units 1 and 2, hydrologic analysis LAR. Given that the version and application of ArcGIS and QGIS software for performing the hydrology analysis for WBN, Units 1 and 2, are the same as those used for SQN, Units 1 and 2, the NRC staff finds that design verification activities for ArcGIS and QGIS meet the requirements in Criterion III and Criterion VII of Appendix B to 10 CFR Part 50.

CEII - DO NOT RELEASE

CEII - DO NOT RELEASE CEII - DO NOT RELEASE 3.10.2.3 Python Scripts for Running the PMP Evaluation Tool and ArcGIS and QGIS software libraries and Microsoft Excel Templates Attachment A of Enclosure 5 of the LAR submittal states, in part, that automation routines were developed utilizing both the Visual Basic Applications (VBA) and Python programming language to run the PMP Evaluation Tool and perform subsequent ArcGIS processing to produce weighted average PMP depths over model sub-basins. A template Microsoft Excel workbook (using VBA) was developed to execute the PMP Evaluation Tool, write a project PMP specific Python script for ArcGIS processing, execute the Python script, and retrieve/store ArcGIS computed weighted average depths. A separate Python script was developed to allow GIS processing of the gridded precipitation within the QGIS software environment. The licensee verified these Python scripts using design review, and the template Microsoft Excel workbook application was verified using hand calculations.

The NRC staff evaluated the information presented in the WBN, Units 1 and 2, hydrologic analysis LAR and the RAI response for validating the python scripts and Microsoft Excel templates created for automating the PMP computation process. The staff determined that the technical basis for the staffs conclusion in section 2.3.3 of the SE for the SQN, Units 1 and 2, hydrologic analysis LAR (Reference 0) with respect to the validation of the Python scripts and Microsoft Excel templates is the same as the information presented in the WBN, Units 1 and 2, hydrologic analysis LAR. Given that the Python scripts and Microsoft Excel templates used to automate the PMP computation process for WBN, Units 1 and 2, are the same as those used for SQN, Units 1 and 2, the NRC staff finds that the requirement for establishing design control measures to verify the adequacy of design in Criterion III of Appendix B to 10 CFR Part 50 has been met for these tools.

3.10.3 Technical Conclusion for Software Dedication The NRC staff reviewed the licensees quality assurance (QA) portion of the software tools described in the submittal for the WBN, Units 1 and 2, hydrologic analysis LAR, including applicable information submitted in the SQN, Units 1 and 2, hydrologic analysis LAR. Given that the software tools used to perform the hydrology analysis for WBN, Units 1 and 2, are the same as those for SQN, Units 1 and 2, the basis for the NRC staffs approval of the QA portion of the software tools in the SQN, Units 1 and 2, hydrology analysis is applicable for the WBN, Units 1 and 2, hydrology analysis LAR. As such, the staff determined the QA activities performed to qualify these tools for use in the WBN Units 1 and 2 hydrologic analyses meet the applicable requirements of Appendix B to 10 CFR Part 50. Specifically, the staff concludes that the commercial-grade dedication activities and results of the PMP Evaluation Tool meet the requirements of Criterion II, III, and VII of Appendix B to 10 CFR Part 50. The staff also concludes that the design review activities performed for ArcGIS and the Python scripts and Microsoft Excel templates created to automate the PMP computation process meet the requirements of Criterion III of Appendix B to 10 CFR Part 50. Therefore, the NRC staff concludes that the QA portion of the WBN, Units 1 and 2, hydrologic analysis LAR is acceptable.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE

4.0 STATE CONSULTATION

In accordance with the Commissions regulations, the Tennessee State official was notified of the proposed issuance of the amendment on January 14, 2025. The State official had no comments.

5.0 ENVIRONMENTAL CONSIDERATION

The amendments change requirements with respect to the installation or use of facility components located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission previously issued a proposed finding that the amendment involves no significant hazards consideration published in the Federal Register on April 2, 2024 (89 FR 22754), and there has been no public comment on such finding. Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.

6.0 CONCLUSION

The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) there is reasonable assurance that such activities will be conducted in compliance with the Commissions regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.

7.0 REFERENCES

1. Hulvey, Kimberly, Tennessee Valley Authority (TVA), letter to the NRC, Application to Revise Watts Bar Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (WBN-19-011), December 26, 2023, Agencywide Documents Access and Management System Accession No. ML24003A270.

2. Hulvey, Kimberly, TVA, letter to NRC, Response to Request for Additional Information Regarding Application to Revise Watts Bar Nuclear Plant, Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (WBN-19-011)

(EPID L-2024-LLA-0006), November 20, 2024, (ML24325A470).

3. Tennessee Valley Authority, Topical Report TVA-NPG-AWA16-A, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041, Revision 1, (ML19155A047).

4. U.S. Nuclear Regulatory Commission, Japan Lessons-Learned Project Directorate (JLD)

Interim Staff Guidance (ISG), JLD-ISG-2013-01, Guidance For Assessment of Flooding Hazards Due to Dam Failure, Revision 0, July 29, 2013, (ML13151A153).

5. U.S. Army Corps of Engineers, Hydrologic Engineering Center, River Analysis System, HEC-RAS computer software, version 3.1.3.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE

6. Shea, J. W., TVA, letter CNL-16-136 to NRC, Request for Review and Approval of Topical Report TVA-NPG-AWA16, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041, September 20, 2016, (ML16264A454).

7. Henderson, E. K., TVA, letter CNL-18-081 to NRC, Request for Review and Approval of Topical Report TVA-NPG-AWA16, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041, Revision 1, June 22, 2018, (ML18192A510).

8. Shoop, U., NRC, letter to Shea, J. W., TVA, Final Safety Evaluation Report for Tennessee Valley Authority Topical Report TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis Calculation CDQ0000002016000041, Rev. 1 (EPID-2016-TOP-0011), March 18, 2019, (ML19010A212).

9. Henderson, E. K., TVA, letter CNL-19-040 to NRC, Submittal of Topical Report TVA-NPG-AWA16-A, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis, Calculation CDQ0000002016000041, Revision 1 (EPID L-2016-TOP-0011), May 21, 2019, (ML19155A047).

10. Shoop, U., NRC, letter to Shea, J. W., TVA, Verification Letter of the Approval Version of Tennessee Valley Authority Topical Report TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis Calculation CDQ0000002016000041, Revision 1 (EPID L-2016-TOP-0011), July 3, 2019, (ML19158A395).

11. Tennessee Valley Authority, Watts Bar Nuclear Plant (WBN) Dual Unit Updated Final Safety Analysis Report (UFSAR) Amendment 5, November 8, 2023, (ML23346A225).

12. Leeds, E. J. and Johnson, M. R., NRC, letter to All Power Reactor Licensees and Holders of Construction Permits in Active or Deferred Status, Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3, and 9.3, of the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident, March 12, 2012, (ML12053A340).

13. SECY-11-0093, Recommendations for Enhancing Reactor Safety in the 21st Century, the Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident (ML111861807).

14. NUREG/CR-7046, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America, November 2011, (ML11321A195).

15. Tennessee Valley Authority, Calculation CDQ0000002013000163, Watts Bar Local Intense Precipitation Evaluation - SAR Design Basis (Case 1) And Flood Hazard Re-Evaluation Report (Case 2).

16. National Oceanic and Atmospheric Administration, Automated Surface Observing System (ASOS) Users Guide, March 1998.

17. U.S. Army Corps of Engineers, ETL 1110-2-588, October 2020.

CEII - DO NOT RELEASE CEII - DO NOT RELEASE

18. Tennessee Valley Authority, Calculation CDQ0000002014000033, Wind Waves for Combined-Effect Floods, Revision 004.

19. Tennessee Valley Authority, Calculation CDQ0000002015000044, Gridded Probable Maximum Precipitation Development, Revision 000.

20. Tennessee Valley Authority, Calculation CDQ0000002016000041, TVA Overall Basin Probable Maximum Precipitation and Local Precipitation Analysis, Revision 001.

21. Tennessee Valley Authority, Calculation CDQ0000002016000043, Controlling Storm Type and Week Analysis.

22. Tennessee Valley Authority, Calculation CDQ0000002015000034, Design Storm Temporal Rain Distribution.

23. Tennessee Valley Authority, Calculation CDQ0000002017000080, PMP/PMF Calculation Automation Tool (PCAT) Model Setup.

24. Tennessee Valley Authority, Calculation CDQ0000002017000062, Fort Loudoun Dam and Tellico Dam Probable Maximum Flood (PMF) Analysis.

25. Tennessee Valley Authority, Calculation CDQ0000002014000021, HEC-RAS Model Setup.

26. Tennessee Valley Authority, Calculation CDQ0000002017000059, Chickamauga Dam PMF, Revision R001.

27. Tennessee Valley Authority, Calculation CDQ0000002017000070, Probable Maximum Flood (PMF) Results Summary, Revision 2.

28. Polickoski, J. T., TVA, letter CNL-19-066 to NRC, Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, (TS-19-02), January 14, 2020, (ML20016A396).

29. Buckberg, P. H., NRC, letter to Barstow, J., TVA, Sequoyah Nuclear Plant, Units 1 and 2 -

Issuance of Amendment Nos. 367 and 361 Regarding Changes to the Hydrologic Analyses (EPID L-2020-LLA-0004), March 26, 2024, (ML24040A206).

30. Barstow, J., TVA, letter CNL-20-032 to NRC, Application to Revise Sequoyah Nuclear Plant Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis - Response to Request for Additional Information (TS-19-02) (EPID L-2020-LLA-0004), May 14, 2020, (ML20135H067).

31. Polickoski, J. T., TVA, letter CNL-21-095 to NRC, Application to Revise Sequoyah Nuclear Plant, Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis - Software Dedication Report 16 Update to Revision 4 (TS-19-02)

(EPID L-2020-LLA-0004), December 21, 2021, (package ML22013A280, non-publicly available).

Principal Contributors: N. Tiruneh D. Zhang Date: May 8, 2025